The present invention relates to a compression self-ignition type gasoline engine which selectively executes spark-ignition combustion and self-ignition combustion according to an engine operating condition, and more particularly to a control system for controlling such a compression self-ignition type gasoline engine.
In a compression self-ignition combustion process, combustions start at plural points in a combustion chamber, and thereby a high combustion speed is obtained. Further, such compression self-ignition combustion ensures stable combustion even under a lean air-fuel ratio condition, as compared with the spark-ignition combustion, and thereby improves fuel consumption. Furthermore, since the combustion temperature of the compression self-ignition combustion is lower than that of the spark-ignition combustion, it is possible to largely decrease NOx in exhaust gases. Additionally, if fuel and air are previously and sufficiently mixed with each other before the compression self-ignition, NOx is further decreased.
However, such self-ignition combustion is largely affected by a combustion parameter so that if the combustion parameter is offset from a target combustion parameter, the self-ignition combustion generates knocking or unstable combustion and is affected by aging or dispersion of the engine in production. Although Japanese Patent Provisional Publication No. H8-151951 discloses an engine control system which is arranged to vary a correction value for a knocking level and a correction value for an ignition timing in order to respond to the dispersion or aging of a spark-ignition type engine or a knock sensor thereof, a self-ignition type gasoline engine has to be controlled on taking account of knocking and combustion stability during the self-ignition combustion. Therefore, even if the mentioned art is simply adapted to the self-ignition type gasoline engine, there yet remain several problems in control of the self-ignition combustion.
It is an object of the present invention to provide a control system which highly and stably controls an efficiency of self-ignition combustion of a compression self-ignition gasoline engine regardless the aging or dispersion of engines in production.
A control system of a self-ignition gasoline engine in accordance with the present invention is arranged to change a combustion condition of the engine between a self-ignition combustion and a spark-ignition combustion according to an operating condition of the engine. The control system comprises a combustion condition detector detecting a combustion condition in the engine and an engine control unit connected to the combustion condition detector. The engine control unit detects a self-ignition limit of a self-ignition combustion region on the basis of the combustion condition, and varies a combustion parameter during the operation of the self-ignition combustion so that the combustion condition approaches the self-ignition combustion limit and the self-ignition combustion operation is executed under a condition maintaining the self-ignition limit.